We don't currently have an option like that. The "join string" comes directly from MusicBrainz, and it's usually something like "and" or "feat.".

However, this is a fortuitous time to be asking about this. Recently, the MB style guide changed to allow a broader variety of join strings, so the "client side" has been asked to provide ways to normalize this for users. Here's the discussion regarding Picard:github.com/metabrainz/picard-plugins

Great! Actually, I meant to ask you the favor of opening an issue in our GitHub repository (i.e., the beets repository). If you don't mind giving some context by linking to the Picard issue too, that would be awesome!